The present invention relates to devices for testing acoustic transducers, and particularly devices for the in situ testing of hydrophones mounted on seismic streamer cables.
In seismic exploration a long seismic streamer cable, for example, two to three miles long, is towed behind the survey vessel. The cable contains hydrophones which are spaced along the cable for receiving the pressure waves that are created by the seismic source. The seismic cable also has a stress member and suitable electrical leads for transmitting the hydrophones' signals to the exploration vessel. The complete assembly is enclosed in a hollow tube which is filled with a fluid such as kerosene or the like to provide neutral buoyancy for the cable. As can be appreciated, these cables are subject to damage and possible rupture of the tubular member. In case of a rupture, the fluid filling the cable is lost and the cable will sink. In addition, the sea water will invade the cable and short the electrical circuits in the cable.
Recently, developments have been undertaken to provide a semi-rigid foam-filled cable which would replace the liquid-filled cable described above. The foam-filled cable will not sink in case of damage to the outer covering, even including partial damage to the foam filling. In the case of foam-filled cables, the mounting of the hydrophones on the cable, and their operation, is more critical than in the case of the liquid-filled cables. The problem arises from the requirement that the hydrophones be placed on the outside of the cable so that they can receive the pressure waves. This increases the chances of the hydrophones being damaged from handling the cable and while towing it through the water. In both foam-filled and liquid-filled cables, it would be desirable to test the hydrophones for proper operation after they have been installed in the cable. As explained, it is more critical in the case of foam-filled cables that one have the capability of testing the individual hydrophones for proper operation.
In the past, hydrophone testing apparatus has comprised a liquid-filled tank or container either open to the atmosphere or pressurized to simulate a submerged hydrophone condition. In either case the containers are normally limited in size so that it is impossible to place the complete seismic cable in the container and check the hydrophones in place. Thus, the practice has been to check individual hydrophones prior to mounting them in the cable. As explained above, it is desirable in the case of foam-filled cables to check for proper operation of the hydrophones with the hydrophones mounted on the cable.
In addition to the above problems, it is also desirable that one be able to check the hydrophones for proper operation in the field. This requires that the test apparatus be capable of use with the hydrophones mounted on the cable. Also, the test apparatus must be portable and simple to use since a large number of hydrophones must be tested.